The present invention relates to a surface elastic wave filter, and more particularly to a filter employing a surface elastic wave device comprising electrodes in the form of a pair of thin metallic films on a surface of a piezoelectric body for sending and receiving a surface elastic wave between the electrodes to pick up a signal having desired frequency characteristics.
Surface elastic wave devices utilize the properties of a surface elastic wave such that most of its energy is propagated along a surface of a solid body. Such surface elastic wave devices are used in oscillators, filter circuits, delay circuits, and the like in various pieces of communication or electronic equipment. The surface elastic wave device comprises, for example, a piezoelectric crystal body with a ground surface and electrodes in the form of a pair of thin metallic films on the surface of the piezoelectric crystal body for sending and receiving a surface elastic wave between the electrodes. More specifically, when one of the electrodes or the piezoelectric crystal body receives an electric input signal, the piezoelectric body vibrates due to the piezoelectric effect, and the vibration is transmitted from the electrode and propagated as an elastic wave along the surface of the piezoelectric body toward the other electrode. The other electrode receives the elastic wave and issues an electric output signal due to the reverse piezoelectric effect of the piezoelectric body.
FIG. 1 of the accompanying drawings illustrates a conventional filter, generally designated by the reference numeral 2, comprising such a surface elastic wave device. The filter 2 has a piezoelectric body 4 serving as a signal propagation medium and a pair of input and output electrodes 6, 8 disposed as signal transducer means on a surface of the piezoelectric body 4. The input electrode 6 comprises a pair of common electrodes 12a, 12b having plural parallel electrode fingers 10a, 10b, respectively, in the shape of comb teeth. The electrode fingers 10a, 10b are disposed in interdigitating relation. An input signal IN is applied between the common electrodes 12a, 12b. The output electrode 8 comprises a pair of common electrodes 16a, 16b having plural interdigitating parallel electrode fingers 14a, 14b, respectively. An output signal OUT is produced from between the common electrodes 16a, 16b. The electrode fingers 10a, 10b and 14a, 14b extend perpendicularly to the direction indicated by the arrow T along which a surface elastic wave is propagated on the surface of the piezoelectric body 4.
When an input signal IN is applied between the common electrodes 12a, 12b of the input electrode 12, a surface elastic wave is generated between the electrode fingers 10a, 10b due to the piezoelectric effect. The produced surface elastic wave is propagated in the direction T and reaches the output electrode 8, which converts the surface elastic wave to an electric signal due to the reverse piezoelectric effect. The electric signal is issued as an output signal OUT from between the common electrodes 16a, 16b of the output electrode 16.
It is known that the output signal OUT from the filter 2 has frequency characteristics dependent on the structure of the input and output electrodes 6, 8. Therefore, an output signal OUT of desired frequency characteristics might be obtained from the filter 2 by adjusting or varying the spacings between the electrode fingers 10a, 10b, 14a, 14b or the number or the configuration of these electrode fingers. It would, however, be quite difficult to achieve complex frequency characteristics having a plurality of passbands or notch filter characteristics having a frequency rejection band within a passband in the above filter, i.e., a filter including in combination an apodized electrode and a normal electrode.
Another conventional surface elastic wave filter is shown in FIG. 2 of the accompanying drawings The filter, generally denoted at 18, comprises an input electrode 20 and an output electrode 22 on a surface of a piezoelectric body 4. The input electrode 20 has two common electrodes 24a, 24b. The common electrode 24a has a plurality of electrode fingers 26a in the shape of comb teeth which are progressively closer to each other toward the other common electrode 24b. The common electrode 24b has a plurality of electrode fingers 26b in the shape of comb teeth which are progressively spread away each other toward the common electrode 24a. These electrode fingers 26a, 26b are disposed in interdigitating fashion. The output electrode 22 includes a pair of common electrodes 28a, 28b having electrode fingers 30a, 30b, respectively, which are shaped and positioned in the same manner as the electrode fingers 26a, 26b of the input electrode 20.
It is known in the art that the frequency characteristics of an output signal OUT produced by the filter 18 have a substantially rectangular pattern, as shown in FIG. 3. Denoted at a in FIG. 3 is the width of a passband in which the output signal 18 is produced by the filter 18, the passband width a being determined by the maximum and minimum spacings between the electrode fingers 26a, 26b or 30a, 30b.
Since the frequency characteristics of the output signal OUT produced by the filter 18 is of a simple rectangular pattern, the electrodes need to be designed further in order to obtain an output signal OUT which has desired complicated frequency characteristics. Obtaining an output signal OUT which has desired complicated frequency characteristics might be accomplished by combining the filter 2 or the filter 8 with a filter in the form of an electric circuit comprising resistors, capacitors, and the like. However, use of such an external electric circuit would make the entire assembly more complex.
The filter is required to increase its ability to separate a desired signal from an unwanted signal, known as "selectivity", by increasing the ratio S (see FIG. 3) between the insertion loss at passed frequencies and the loss at cut-off frequencies. With a generally employed combination of an apodized electrode and a normal electrode, however, it would be difficult to improve selectivity with respect to wideband filters having a fractional band of 30% or more. It would be possible to increase the fractional band, but no method has been proposed to better the selectivity, for the filter as illustrated in FIG. 2.
No proposal has been made to increase the selectivity with respect to the control of frequency characteristics on the propagation path between the input and output electrodes. Therefore, in applications such for example as a notch filter having a frequency rejection band within a passband, it has been difficult to design the filter such that the rejection band will be reduced while increasing the amount of attenuation of rejected frequencies. More specifically, the intensity distribution of a surface elastic wave excited when a certain frequency is applied, in a direction normal to the direction of propagation, is expressed as a function of sin(x)/x with its frequency f at the center, where EQU x=N.pi.(f-fi)/fi
(N is the number of electrode finger pairs) Therefore, even if the surface elastic wave is attenuated in a channel in the propagation path which corresponds to a certain frequency, the surface elastic wave is excited from another channel as a side lobe. Consequently, where the number of input electrode finger pairs is equal to the number of output electrode finger pairs, the amount of frequency attenuation attained in a certain channel is only 26 dB even from the standpoint of theoretical considerations.